Although silicon is the most widely used semiconductor at the present time, Group III-V compound semiconductors are used in a wide variety of applications. For example, such semiconductors are widely used in microwave and opto-electronic applications. The devices used in such applications frequently have AlGaAs/GaAs present, and device fabrication frequently requires the selective etching of GaAs with respect to AlGaAs.
Accordingly, highly selective etches have been developed. Probably the most widely used is CCl.sub.2 F.sub.2 or dichlorofluormethane. It is commonly known as Freon-12 and is nontoxic, nonflammable, and noncorrosive. Selectivities range from 200: to 600:1. See, for example, Journal of Vacuum Science and Technology, B9(6), by Cameron et al., Nov/Dec 1991, p. 3538, and Journal of Vacuum Science and Technology, B6(1), by Seabaugh, Jan/Feb 1988, p. 77, for reports of typical results. Although well suited for the etching described, it is a chlorofluorocarbon with a high ozone-depleting potential. As such, its use is undesirable and manufacture in the United States after 1995 is questionable.
He has long been used as an additive in etch chemistries. There are several reasons for adding He to a plasma. For example, it has a high thermal conductivity and may promote etch uniformity. Additionally, it may prevent or reduce polymer formation in C containing etches. For example, Japanese Journal of Applied Physics, 20, by K. Hikosaka et al., November 1981, pp. L847-L850 added He to a CCl.sub.2 F.sub.2 system and observed a decrease in the GaAs etch rate. See, also, Journal of Vacuum Science and Technology, B9(5), by Collot et at., Sep/Oct 1991, pp. 2497-2502, for a report on etch chemistries using He.
There are, of course, many halogen-based species available. For example, hydrochlorofluorocarbons might be used, but they also have an ozone-depleting potential and are therefore undesirable. Still other alternatives mix chlorine- and fluorine-based species or oxygen- and chlorine-based species to obtain selectivity.